The rear wheels are making a scraping sound, which, I believe is an indicator to replace the pads. How does one determine if it is time to replace the rotors?
While the scaping sound is a good indicator, the way to determine for certain is to remove the wheel and measure the rotor thickness with a micrometer. If it measure at or below the minimum service limit (from the SERVICE MANUAL and often embossed in the rotor itself) you should replace them.
Even if they do measure above the service limit, you should inspect for any damage or unusual wear including runout and replace if necessary.
If the pads have gone to metal to metal contact on the rotors, then replace them.
While it may be possible to machine them if they are still thick enough they could also be warped and that needs to be measured with a dial gauge. Even if they can be machined then they can warp after due to the heating / damage from the pads metal to metal contact.